Display devices which have employed a liquid crystal composition have widely been used for watches, desktop calculators, and others. These liquid crystal display devices employ the optical anisotropy and dielectric anisotropy of liquid crystal substances. Liquid crystal phase includes a nematic liquid crystal phase, smectic liquid crystal phase, and cholesteric liquid crystal phase. Among the display devices, ones which have employed a nematic liquid crystal phase have most widely been used practically. Mode of liquid crystal display device includes a TN (twisted nematic) mode, DS (dynamic scattering) mode, guest-host mode, and DAP (Deformation of Aligned Phases) mode corresponding to electro-optic effect employed therein. Liquid crystalline compounds used in the display devices of those modes desirably exhibit a liquid crystal phase in a temperature range as wide as possible and are required to be stable against moisture, heat, and air. Particularly, demand for display devices of active matrix mode typified by Thin Film Transistor (TFT) mode has recently been increased. When liquid crystal panels are optically designed, the product .DELTA.n.multidot.d of the thickness of the cell of a display device (d) and the value of optical anisotropy (.DELTA.n) must be adjusted to a certain value. At that time, since d tends to decrease partly due to the progress in the reduction of cell thickness in TFT mode, liquid crystalline compounds having a large .DELTA.n became necessary. Liquid crystalline compounds are also required to exhibit a liquid crystal phase in a wide temperature range and to have a miscibility with other liquid crystalline compounds as high as possible especially at low temperatures. Whereas many liquid crystalline compounds have already been known, there exists no substance at present which satisfies the conditions mentioned above by a single liquid crystalline compound, and thus several kinds of liquid crystalline compounds and further non-liquid crystalline compounds are mixed together and actually provided for practical uses at present.
Liquid crystalline compounds having chlorine atom as substituent for the purpose of improving characteristics of liquid crystal are known, for example, as follows: ##STR2##
Among these, compound 1) is not good in miscibility with known liquid crystalline compounds at low temperatures, and compounds 2), 3), and 4) are small in optical anisotropy. Whereas compounds 5) and 6) exhibit a liquid crystal phase at a comparatively wide temperature range, they are not good in miscibility with known liquid crystalline compounds at low temperatures as in the case of compound 1). Accordingly, these compounds are not satisfactory as liquid crystal material used for TFT mode.